Cellular base stations typically include, among other things, one or more radios, baseband units, and antennas. The radio receives digital information and control signals from a baseband unit and modulates this information into a radio frequency (“RF”) signal that is then transmitted through an antenna. The radio also receives RF signals from the antenna and demodulates these signals and supplies them to the baseband unit. The baseband unit processes the demodulated signals received from the radio into a format suitable for transmission over a backhaul communications system. The baseband unit also processes signals received from the backhaul communications system and supplies the processed signals to the radio. A power supply may also be provided that generates suitable direct current (“DC”) power signals for powering the baseband unit and the radio.
In order to increase coverage and signal quality, the antennas in many cellular base stations are located at the top of a tower, which may be, for example, about fifty to two hundred feet tall. Conventionally, the power supply, baseband unit and radio were all located in an equipment enclosure at the bottom of the tower to provide easy access for maintenance, repair and/or later upgrades to the equipment. Coaxial cable(s) were routed from the equipment enclosure to the top of the tower. These coaxial cables carry signal transmissions between the radios and the antennas.
FIG. 1 schematically illustrates a conventional cellular base station 10. As shown in FIG. 1, the cellular base station 10 includes an equipment enclosure 20 and a tower 30. A plurality of baseband units 22 and radios 24, and a power supply 26, are located within the equipment enclosure 20 (while three baseband units 22 and radios 24 are illustrated in FIG. 1, it will be appreciated that more or less baseband units 22 and radios 24 may be provided). Three sectorized antennas 32-1, 32-2, 32-3 are located at the top of the tower 30. Three coaxial cables 34 (which are bundled together outside of the enclosure 20 to appear as a single cable) connect the respective radios 24 to the respective antennas 32-1, 32-2, 32-3. Note that herein when multiple units of an element are provided, each individual unit may be referred to individually by the reference numeral for the element followed by a dash and the number for the individual unit (e.g., antenna 32-2), while multiple units of the element may be referred to collectively by their base reference numeral (e.g., the antennas 32).
In recent years, a shift has occurred and the radios 24 are now more typically located at the top of the tower 30 in new or upgraded cellular installations. Radios 24 that are located at the top of the tower 30 are typically referred to as “remote radio heads” 24. Using remote radio heads 24 may significantly improve the quality of the cellular data signals that are transmitted and received by the cellular base station as the use of remote radio heads 24 may reduce signal transmission losses and noise. In particular, the coaxial cables 34 that connect radios 24 at the bottom of a tower 30 to antennas 32 at the top of the tower 30 may be 100-200 feet in length or more. The signal losses that may occur when radio signals at the RF frequencies used by cellular systems (e.g., 1.8 GHz, 3.0 GHz, etc.) are transmitted over such extended lengths of coaxial cable 34 may be very significant, because at these frequencies the coaxial cables 34 exhibit skin effect and dielectric losses which are considerably higher than at low frequencies. Because of this loss in signal power, the signal-to-noise ratio of the RF signals may be degraded in cellular base stations 10 that locate the radios 24 at the bottom of the tower 30 as compared to cellular base stations where remote radio heads 24 are located at the top of the tower 30 next to the antennas 32. The signals are typically transmitted between the baseband units 22 and the remote radio heads 24 in digital form over optical fibers.
FIG. 2 is a schematic diagram that illustrates a cellular base station 10′ according to this newer architecture. As shown in FIG. 2, the baseband units 22 and the power supply 26 may still be located at the bottom of the tower 30 in the equipment enclosure 20. The radios 24 in the form of remote radio heads 24 are located at the top of the tower 30 immediately adjacent to the antennas 32. One or more fiber optic cables 38 that include a plurality of optical fibers connect the baseband units 22 to the remote radio heads 24. Fiber optic links may be provided between the baseband units 22 and the remote radio heads 24 because fiber optic cables may be lighter than coaxial cables, and may provide greater bandwidth and lower loss transmissions.
While the use of tower-mounted remote radio heads 24 and fiber optic cables 38 may increase bandwidth and improve signal quality, this architecture also requires that DC power be delivered to the top of the tower 30 to power the remote radio heads 24 (the antennas 32 may be passive devices that do not require an electrical power feed or may have very low power requirements such that they may be powered by a single small cable that carries control communications to the antennas 32). As shown in FIG. 2, this is typically accomplished by running a separate power cable 36 up the tower 30 that provides a DC power supply signal to the remote radio heads 24. The separate power cable 36 is typically bundled with the fiber optic cable(s) 38 so that they may be routed up the tower 30 together. The bundled cable that includes the power cable 36 and fiber optic cable(s) 38 is typically referred to as a “trunk” cable 40. The end of the trunk cable 40 at the bottom of the tower 30 is terminated into a first breakout box 42-1, and the end of the trunk cable 40 at the top of the tower 30 is terminated into a second breakout box 42-2. In some case, the ends of the trunk cable 40 may be pre-terminated into the respective breakout boxes 42-1, 42-2 at the time of manufacture.
In a typical newly-installed cellular installation, three (or more) antennas 32 are mounted on the tower 30, and six, nine or even twelve remote radio heads 24 may be mounted near the antennas 32 at the top of the tower 30. A first set of jumper cables 46 connect the baseband units 22 and the power supply 26 to the first breakout box 42-1, and a second set of jumper cables 48 connect the second breakout box 42-2 to the remote radio heads 24. Each set of jumper cables 46, 48 may include a plurality of data cables and a plurality of power cables (which in some cases may be combined into a set of composite jumper cables that each include both power and data components). A first end of each jumper cable 46 is terminated into the first breakout box 42-1, and a first end of each jumper cable 48 is terminated into the second breakout box 42-2. At least one jumper cable 48 is connected between the second breakout box 42-2 and each remote radio head 24 to provide power to the remote radio head 24 and to carry uplink and downlink communications between the remote radio head 24 and its associated baseband unit 22 at the bottom of the tower 30.